Intradral radiographic system for substantially confining the x-ray beam to the film

ABSTRACT

An x-ray beam of rectangular cross-section is produced at the end of a lead-lined rectangular collimating tube. The rectangular tube is aligned with a dental film-holding instrument comprising a bite block, a directing rod attached to the bite block, and a rectangular guide member attached to the directing rod. The alignment is achieved by keying the rectangular guide member to the end of the rectangular collimating tube.

United States Patent [191 Updegrave [111 3,745,344 [451 July 10,1973

l INTRADRAL RADIOGRAPHIC SYSTEM FOR SUBSTANTIALLY CONFINING THE X-RAY BEAM TO THE FILM [76] Inventor: William J. Updegrave, 616 W.

Harrter St., Philadelphia, Pa. 191 19 [22] Filed: Dec. 20, 1971 [21] App]. No.: 209,827

[52] US. Cl. 250/70, 250/105 [51] Int. Cl G211 5/04, 1-101j 35/16 [58] Field of Search 250/69, 70, 105

[56] 7 References Cited UNITED STATES PATENTS 1,667,442 4/1928 Sikes 250/70 X 1,947,014 2/1934 Levy 250/70 X 3,102,957 9/1963 Slauson 250/105 3,304,422 2/ 1967 Medwedeff 250/70 3,473,026 10/ 1969 Updegrave 250/70 Primary Examiner-William F. Lindquist Attorney-Robert B. Washbum ABSTRACT An x-ray beam of rectangular cross-section is produced at the end (if a lead-lined rectangular collimating tube. The rectangular tube is aligned with a dental filmholding instrument comprising a bite block, a directing rod attached to the bite block, and a rectangular guide member attached to the directing rod. The alignment is achieved by keying the rectangular guide member to the end of the rectangular collimating'tube.

11 Claims, 6 Drawing Figures Patented July 10, 1973 3,745,344

2 Sheets-Sheet l Patented July 10, 1973 2 Sheets-Sheet 2 INTRADRAL RADIOGRAPHIC SYSTEM FOR SUBSTANTIALLY CONFINING THE X-RAY BEAM TO THE FILM BACKGROUND OF THE INVENTION This invention relates to radiography, and more particularly, to dental or intraoral radiography.

Collimation of an x-ray beam to conform to the size and shape of the x-ray film is a conventional procedure followed in medical radiography. However, most dental radiographic techniques utilize large beams of circular cross-section even though dental films are rectangular in shape and have a smaller surface area than the crosssectional area of the beam. By utilizing a large circular beam, alignment of the beam with the patients teeth and the film is facilitated since there is a larger margin for error. While the use of a large circular beam, e.g., a 4-inch diameter beam, considerably reduces the chances of cone-cutting and thereby eliminates the necessity for exposing a patient to retakes, the patient is exposed to considerable radiation due to the crosssectional area of the large circular beam. Since large diameter beams unnecessarily irradiate tissues far beyond the borders of the dental film, the National Committee on Radiation Protection, in Report No. 35, has recommended that the useful beam for intraoral radiography be restricted to a diameter of not more than 3 inches at the end of the cone.

In order to overcome the alignment difficulties associated with the use of a beam having a smaller diameter, various instruments have been developed to eliminate cone-cutting and establish the correct angulat ion of the beam. In this connection, instruments for practicing the Bisecting-Angle, Right-Angle, Periapical, and Bite-Wing techniques have been described in the following publications: W.J. Updegrave, New Horizons in Periapical Radiography, Rinn Corporation, Elgin,

Ill., 1966; W.J. Updegrave, Simplified and Standard- Dental Radiography-Common Problems and Solutions, Journal Tennessee Dental Association, Vol. 51, No. 3, July, 1971.

Even though these instruments have greatly reduced cone-cutting thereby eliminating the necessity for x-ray retakes, the patient is still subjected to considerably more radiation than is necessary since the size and shape of the beam does v not conform with thesize and shape of the film. In this connection, the National Committee on Radiation Protection has also indicated that it is desirable to limit the useful beam to the smallest area consistent with diagnostic requirement It is acknowledged that film-object-beam alignment procedures are clinically difficult. Continuing efforts devoted to practical means for insuring such alignment ideal. For example, U.S. Pat. Nos. 3,304,422 Medwedeff, 3,304,423 Medwedeff, Re25,773 Medwedeff et al, and 3,469,099 Best et al disclose a film-carrying instrument which is held by patient in his mouth. The instrument includes a shield which is substantially rectangular so as to conformwith the size and shape of the x-ray film. There devices are discussed by Donald D. Weissman and Gerald E. Longhurst in a publication entitled Manual of Rectangular Field Collimation for intraoral Periapical Radiography, 197 1. However, these devices are relatively heavy and cause the patient some discomfort when held in his mouth which can in itself lead to alignment difficulties between the film, the shield opening, and the x-ray tube or cone. Even with the assistance of the patients hand in stabilizing the device, it is difficult to maintain the proper alignment with such an instrument in his mouth assuming that alignment can be initially achieved. Of course, any slight misalignment can result in substantial cone-cutting so as to necessitate retakes and increase the patients exposure to radiation. Finally, clinical application of these instruments is difficult.

Another approach to reducing patient exposure to radiation is described by R. A. Goepp et al in an article entitled The Reduction of Unnecessary X-Ray Exposure During Intraoral Examination; Oral Surgery, 16:39 January, 1963. In that article, a tube or cone having a rectangular orifice is disclosed. The cone which is adapted to be rotatably mounted on an x-ray unit is intended to minimize the exposure of the patient by rectangular collimation of a beam which conforms adds to the difficulty, and there is no means for positively aligning the film-holdinginstrument with the cone.

SUMMARY OF THE INVENTION It is an object of this invention to provide a practical intraoral radiographic system which minimizes the exposure of patients to x-rays.

It is another object of this invention to provide a practical intraoral radiographic system which minimizes patient x-ray exposure by assuring alignment between the x-ray beam and the x-ray film.

It is a further object of this invention to provide a practical intraoral radiographic system which assures proper alignment between an x-ray beam of rectangu lar cross-section and the x-ray film.

It is a still further object of this invention to provide a practical intraoral radiographic system which generally improves the radiographic quality by limiting 'scatter radiation.

It is yet another object of this invention to provide a practical intraoral radiographic system having a simplicity and standarization which routinely produces radiographs of high anatomic fidelity.

It is a further object of this invention to provide a practical intraoral radiographic system, the use of which may be readily and effectivley taught to dentists ancillary personnel.

In accordance with these and other objects, a preferred embodiment of the invention comprises a dental radiographic system including a film-holding instrument having a positioning means comprising a rectangular frame member adapted to extend from the patients mouth and a bite block adapted to be held between the teeth and carry a rectangular intraoral x-ray film. The system further includes a rectangular x-ray beam shaping tube adapted to be mounted on an x-ray generating unit to form an x-ray beam of substantially rectangular cross-section. The positioning means and the beam shaping means may comprise keying means for maintaining the positive alignment between the sides and corners of the rectangular film and the beam of rectangular cross-section through positive engagement between the positioning means and the beam shaping means.

The beam shaping means may comprise a collimator including a lead shield having a rectangular opening and a lead-lined rectangular tube forming a rectangular orifice. By providing an adapter which is mounted on an x-ray generating unit, the rectangular tube may be rotated so as to achieve proper alignment with the x-ray film.

The film-holding instrument itself may comprise the bite block in addition to a guide member having a rectangular opening which is connected to the bite block by a directing rod. The rectangular opening is substantially the same size as the rectangular orifice in the lead-lined rectangular tube.

The keying means may comprise projections which extend inwardly from opposite sides of the rectangular opening in the guide member and notches on opposite sides of the rectangular orifice in the rectangular tube which receive the projections of the guide member.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects of the invention will be better understood from the following description taken in conjunction with the drawings in which:

FIG. 1 is a perspective view of a system embodying the invention;

FIG. 2 is a sectional view of the collimator and the tube positioning member shown in FIG. 1 taken along section line 2.2;

FIG. 3 is a sectional view of the tube positioning member shown in FIG. 1 taken along section line 3--3 with the guide member displaced from the end of the tube;

FIG. 4 is an exploded perspective view of the rectangular collimator;

FIG. 5 is a sectional view of the rectangular collimator shown in FIG. 1 taken along section line 5--5; and

FIG. 6 is a side view of another system embodying the invention shown in use in the mouth of a ptient.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT An intraoral radiographic system in accordance with this invention is shown inFIG. l as comprising a film holding instrument l0 and an x-ray beam shaping means in the form ofa rectangular collimating tube 12 attached to a dental x-ray unit not shown. The film holding instrument comprises a bite block 14 adapted to be held between a patients teeth and support a rectangular piece of dental film 16 within the patients mouth. The film holding instrument 10 further comprises a directing rod 18 adapted to extend from the patients mouth while being removably attached to the bite block 14 and a tube positioning member 20 having a rectangular opening of substantially the same size and shape as the orifice of the rectangular collimating tube 12. This sameness or similarity in size and shape assists in achieving alignment when the member 20 is coupled to the tube 12.

In accordance with one very important aspect of the invention, keying means 22 located at the tube positioning member 20 and the end of the tube 12 are provided to assure, by means of positive engagement alignment between the rectangular opening in the tube positioning member 20 and the rectangular orifice in the rectangular tube 12. This assures alignment between the sides and corners of the rectangular film l6 and the x-ray beam of rectangular cross-section which emanates from the end of the tube 12 since the spatial relationship between the tube positioning member 20 and the bite block 14 is fixed except for the distance between the tube positioning member 20 and the dental film 16. This distance depends upon which of the adjacent apertures 24 in the bite block 14 receives the two forked ends 26 of the directing rod 18. This distance also depends upon the position of the tube positioning member 20 on the directing rod 18. Note that the directing rod is square in cross-section as is the rod receiving aperture 28 in the tube positioning member 20 to preclude rotation of the tube positioning member 20 the tube positioning member 20 will engage the keying means comprising notches 30 which are located on the tube 12. However, if the tube positioning member 20 were inverted on the directing rod 18 along with the bite block 14, the other projection on each side of the rectangular opening in the tube positioning member 18 would engage the respective notches in the tube 12.

FIG. 3 clearly shows the tapered ends 32 of the projections 30. The tapering of these ends limits the amount of material in the path of the x-ray beam as it passes through the rectangular opening in the member 18 thereby limiting the distortion in the radiograph.

In accordance with another important aspect of the invention, the end of the tube 12 adjacent the x-ray unit comprises a rotary adapter 34 as shown in FIGS. 1 and 4. The adapter includes, a male threaded portion 26 which is secured to a lead-lined plastic tube 38 by screws 40 inserted through apertures 42 in the tube 38 and apertures 44 in a partial collar 46. The remainder of the rotary adapter 34 comprises a female threaded member 48 which receives and threadedly engages the male threaded member 36. Note the use of a bayonettype latching means for mounting the female threaded member 48 on the dental x-ray unit not shown. This rotary adapter permits the rectangular orifice of the tube 12 to be aligned with the rectangular opening in the tube positioning member 18 without requiring the pa tient to rotate his head by merely rotating the female member 36 in the male member 48. Furthermore, the rotary adapter permits the rectangular orifice in the tube 12 to be rotated 90 so as to be aligned with a rectangular piece of xray film which is mounted on end in a bite block as will be subsequently described with respect to FIG. 6 rather than on its side as indicated in FIG. 1.

In accordance with another important aspect of the invention, the tube 12 provides double rectangular collimation of the x-ray beam. In this connection, a lead diaphragm 50 is provided at the end of the plastic tube and attached to the male portion 48 of the rotary adapter 34. The lead diaphragm has a rectangular opening so as to provide the collimation of a rectangular beam. In addition, the walls of the plastic tube 38 are lined with lead 52 as shown in FIG. 5 to assure an x-ray beam of rectangular cross-section. It has been found particularly desirable to utilize a lead lined tube 38 which will provide a rectangular orifice of the order of l-% inches by l-% inches. Such an orifice can then be utilized in conjunction with dental x-ray filmhaving dimensions of 1-54 inches by l% inches.

'It will be understood that the length of the tube 38 may vary depending upon the nature of the radiograph being taken. In general, two lengths are utilized, one establishing an 8 inch focal-film distance and the other a 16 inch focal-film distance. The right angle technique illustrated in FIG. 1 would require the longer tube while a bisecting angle technique involving the use of a suitable bite block such as that shown in U.S. Pat. No. 3,473,026 Updegrave, which is incorporated herein by reference, would require the shorter tube. Boththe longer and shorter tubes may be utilized with a bitewing instrument. Suitable bite blocks for all of these techniques are well illustrated in the aforesaid publications entitled New Horizons in Radiography and Simplification and Standarization of the Bisecting-Angle and lnterproximal Technics which are incorporated herein by reference.

In the right angle technique illustrated in FIG. 1, the objective is to maintain the film 16 parallel with the long axis of the teeth being radiographed. Accordingly, the bite blocks for this technique are designed with a long biting surface 54 and a film backing support 56 set at right angles to the biting surface 54. This makes it possibleto position the film as far lingually as necessary .to attain this parallel relationship. The bite block of FIG. 1 is suitable for use in taking posterior radiographs and the film backing portion is accordingly wider than it is tall to permit the x-ray film to rest along one side. In contrast, the bite block of FIG. 6 which is shown as inserted in a patients mouth comprises a film backing support 58 which is taller than it is wide. In this arrangement the film 60 is supported in a groove 62 along one end.

In addition to the difference in the bite blocks, the tube positioning members as well as the directing rods of FIGS. 1 and 6 differ to assure alignment between the rectangular opening in the tube positioning member and dental x-ray film carried by the bite block. Note that the tube positioning member 64 comprises a single projection 66 on each of the opposite sides of the rectangular opening in the tube positioning member which is received by a notch. Only a single projection 66 is required on each side of the rectangular opening since it is not necessary to reverse the member 64 on the directing rod 68. It will be understood that the rectangular opening in the member 64 also stands on end. This, of course, requires rotation of the adapter 36 not shown in FIG. 6.

In the foregoing, a system has been described which achieves a substantial reduction in. irradiation of patient tissue. It has been found that up to 58 percent less tissue is irradiated per dental exposure with rectangular beam collimation as compared with conventional circular beam collimation. The accuracy of alignment between the x-ray beam and the film as accomplished by this system also eliminates the necessity for retake radiographs.

Although a specific embodiment of the invention has been shown and described, it will be understood that various modifications may be made as will fall within the scope of the invention as set forth in the appended claims What is claimed is:

l. A dental radiographic system for limiting patient x-ray exposure comprising:

an x-ray beam shaping means for forming an x-ray beam of substantially rectangular cross-section including a lead-lined tube having a pair of shorter walls and a pair of longer walls forming an axially extending orifice of substantially rectangular crosssection, a lead diaphragm having a substantially rectangular aperture rigidly attached to said tube near one end thereof such that the rectangular orientation of said tube orifice corresponds with the rectangular orientation of said diaphragm aperture, and a flange extending outwardly from said tube at said one end, and terminating in a male circular portion, the other end of said tube including a pair of notches locatedat the ends of said shorter walls;

a substantially circular rotary adapter including a female portion rotatably receiving said circular male portion of said flange and means for mounting said adapter on an x-ray unit; film-holding instrument freely movable with re spect to said x-ray beam shaping means comprising a unitary bite block having a bite portion and a film-backing portion disposed at oneend of said bite portion and projecting upwardly therefrom, said bite portion including roughened upper and lowersurfaces adapted to be held between the teeth, said bite portion further including filmholding slot'located at the junction of said bite portion and said film-backing portion for receiving rectangularly shaped film therein,'said bite portion including a plurality of substantially cylindrical holes extending into said bite block in a direction parallel with said slot and an alignment rod having a first forked portion adapted to be detachably received by said cylindr'ical holes in said bite block, a second portion extending away from said bite block in a direction generally parallel to the longer walls of said orifice in said tube and a third portion extending generally parallel with the axis of said tube and being substantially square in cross-section; and positioning member comprising a rectangular frame member having a pair of shorter sides and longer sides forming a rectangular opening of substantially the same rectangular proportions as said orifice of said tube, an arm extending away from said frame member in a direction generally parallel with the longer sides of said frame member and a coupling portion attached to said arm, said coupling portion having an opening slidably receiving said third portion of said alignment rod such that the long dimension of the film in said slot is generally parallel with the longer sides of said frame member, said rectangular frame member including a pair of projections extending outwardly from said shorter sides and toward the center of said rectangular opening, said projections being adapted to be received by said notches located at the end of said walls of said tube when said rectangular frame member is aligned with said tube so as to assist in aligning said tube, said frame member and said film and thereby substantially confine said beam to strike said film so as to avoid unnecessary x-ray exposure of the patient.

2. A dental radiographic system for limiting patient x-ray exposure comprising:

an x-ray beam shaping means for forming an x-ray beam of substantially rectangular cross-section including a lead-lined tube having a pair of shorter walls and a pair of longer walls forming an axially extending orifice of substantially rectangular crosssection, a lead diaphragm having a substantially rectangular aperture rigidly attached to said tube near one end thereof such that the rectangular orientation of said tube orifice is the same as the rectangular orientation of said diaphragm aperture;

a rotary adapter for mounting on an x-ray unit, said rotary adapter being attached to said one end of said x-ray shaping means so as to permit rotation of said tube and said diaphragm; film-holding instrument freely movable with respect to said xray beam shaping means comprising a bite block having a bite portion and a filmbacking portion projecting generally upwardly from said bite portion, said bite portion including roughened upper and lower bite surfaces adapted to be held between the teeth of the patient, said bite block further comprising a film-holding slot located at the junction of said bite portion and said film-backing portion for receiving rectangular-shaped film therein,

an alignment means connected to said bite block and adapted to extend away from the patient toward said x-ray beam shaping means; and

a positioning member comprising a frame member having a pair of shorter sides and a pair of longer sides forming a rectangular opening of substantially the same rectangular proportions as said orifice of said tube and a coupling portion connected to said alignment means such that the long dimension of the film in said slot is generally parallel with the longer sides of said frame member, said rectangular frame member being adapted to be coupled to the other end of said tube so as to provide a visual aid in establishing alignment between and the same rectangular orientation for said beam rectangular cross-section and said film and thereby substantially confine said beam to strike said film so as to avoid unnecessary x-ray exposure of the patient.

3. The dental radiographic system of claim 2 wherein said alignment means comprises a rod attached to said bite block.

4. The dental radiographic system of claim 3 wherein said coupling portion of said positioning member slidably engages said rod along a portion thereof.

5. A dental radiographic system for limiting patient x-ray exposure comprising:

an x-ray beam shaping means comprising a radioopaque tube having an axially extending orifice of substantially rectangular cross-section;

an adapter including first means for mounting said adapter on an x-ray generating unit and second means for rotatably engaging said x-ray beam shaping means so as to permit rotation of said tube about the axis thereof during x-ray beam orientation; and film-holding instrument freely movable with respect to said tube, said instrument including a bite block adapted to be held between the teeth of the patient and carry a rectangular piece of x-ray-film of substantially the same rectangular proportions as said rectangular cross-section of said orifice, an alignment means connected to said bite block and adapted to extend away from the patient and a positioning means connected to said alignment means a spaced distance from said bite block and having a rectangular frame with a rectangular opening of substantially the same rectangular proportions and orientation as said film such that the long sides of said rectangular film are substantially parallel to the long sides of said rectangular opening and the short sides of said rectangular film are substantially parallel to the short sides of said rectangular opening, said positioning means adapted to be coupled to the end of said tube opposite said adapter to assist in establishing alignment between and the same rectangular orientation for said orifice crosssection, said rectangular opening in said positioning means and said film as said tube is rotated thereby substantially confining the beam to strike said film to avoid unnecessary x-ray exposure ,of the patient.

6. The dental radiographic system of claim 5 wherein said positioning means comprises keying means for maintaining alignment between the sides and corners of said rectangular frame and said tube orifice rectangular cross-section through positive engagement between said positioning means and said beam shaping means.

7. The dental radiographic system of claim 5 wherein said x-ray beam shaping means comprises keying means for maintaining alignment between the sidesand corners of said rectangular frame and said tube orifice rectangular cross-section through positive engagement between said positioning means and said beam shaping means.

8. The dental radiographic system of claim 5 wherein both said x-ray beam shaping means and said positioning means comprises keying means for maintaining alignment between the sides and corners of said rectangular frame and said tube orifice rectangular crosssection through positive engagement between said positioning means and said beam shaping means.

9. The dental radiographic system of claim 8 wherein said keying means comprises at least one notch in said tube and at least one projection extending from said frame member and adapted to be received by said notch to provide alignment between said rectangular opening in said frame member and said tube orifice rectangular cross-section.

wherein said x-ray beam shaping means further comprises a lead diaphragm having a rectangular opening at the end of said tube remote from the patient to form a beam of rectangular cross-section and said tube comprises a lead lining. 

1. A dental radiographic system for limiting patient x-ray exposure comprising: an x-ray beam shaping means for forming an x-ray beam of substantially rectangular cross-section including a lead-lined tube having a pair of shorter walls and a pair of longer walls forming an axially extending orifice of substantially rectangular cross-section, a lead diaphragm having a substantially rectangular aperture rigidly attached to said tube near one end thereof such that the rectangular orientation of said tube orifice corresponds with the rectangular orientation of said diaphragm aperture, and a flange extending outwardly from said tube at said one end, and terminating in a male circular portion, the other end of said tube including a pair of notches located at the ends of said shorter walls; a substantially circular rotary adapter including a female portion rotatably receiving said circular male portion of said flange and means for mounting said adapter on an x-ray unit; a film-holding instrument freely movable with respect to said xray beam shaping means comprising a unitary bite block having a bite portion and a film-backing portion disposed at one end of said bite portion and projecting upwardly therefrom, said bite portion including roughened upper and lower surfaces adapted to be held between the teeth, said bite portion further including film-holding slot located at the junction of said bite portion and said film-backing portion for receiving rectangularly shaped film therein, said bite portion including a plurality of substantially cylindrical holes extending into said bite block in a direction parallel with said slot and an alignment rod having a first forked portion adapted to be detachably received by said cylindrical holes in said bite block, a second portion extending away from said bite block in a direction generally parallel to the longer walls of said orifice in said tube and a third portion extending generally parallel with the axis of said tube and being substantially square in cross-section; and a positioning member comprising a rectangular frame member having a pair of shorter sides and longer sides forming a rectangular opening of substantially the same rectangular proportions as said orifice of said tube, an arm extending away from said frame member in a direction generally parallel with the longer sides of said frame member and a coupling portion attached to said arm, said coupling portion having an opening slidably receiving said third portion of said alignment rod such that the long dimension of the film in said slot is generally parallel with the longer sides of said frame member, said rectangular frame member including a pair of projections extending outwardly from said shorter sides and toward the center of said rectangular opening, said projections being adapted to be received by said notches located at the end of said walls of said tube when said rectangular frame member is aligned with said tube so as to assist in aligning said tube, said frame member and said film and thereby substantially confine said beam to strike said film so as to avoid unnecessary x-ray exposure of the patient.
 2. A dental radiographic system for limiting patient x-ray exposure comprising: an x-ray beam shaping means for forming an x-ray beam of substantially rectangular cross-section including a lead-lined tube having a pair of shorter walls and a pair of longer walls forming an axially extending orifice of substantially rectangular cross-section, a lead diaphragm having a substantially rectangular aperture rigidly attached to said tube near one end thereof such that the rectangular orientation of said tube orifice is the same as the rectangular orientation of said diaphragm aperture; a rotary adapter for mounting on an x-ray unit, said rotary adapter being attached to said one end of said x-ray shaping means so as to permit rotation of said tube and said diaphragm; a film-holding instrument freely movable with respect to said x-ray beam shaping means comprising a bite block having a bite portion and a film-backing portion projecting generally upwardly from said bite portion, said bite portion including roughened upper and lower bite surfaces adapted to be held between the teeth of the patient, said bite block further comprising a film-holding slot located at the junction of said bite portion and said film-backing portion for receiving rectangular-shaped film therein, an alignment means connected to said bite block and adapted to extend away from the patient toward said x-ray beam shaping means; and a positioning member comprising a frame member having a pair of shorter sides and a pair of longer sides forming a rectangular opening of substantially the same rectangular proportions as said orifice of saiD tube and a coupling portion connected to said alignment means such that the long dimension of the film in said slot is generally parallel with the longer sides of said frame member, said rectangular frame member being adapted to be coupled to the other end of said tube so as to provide a visual aid in establishing alignment between and the same rectangular orientation for said beam rectangular cross-section and said film and thereby substantially confine said beam to strike said film so as to avoid unnecessary x-ray exposure of the patient.
 3. The dental radiographic system of claim 2 wherein said alignment means comprises a rod attached to said bite block.
 4. The dental radiographic system of claim 3 wherein said coupling portion of said positioning member slidably engages said rod along a portion thereof.
 5. A dental radiographic system for limiting patient x-ray exposure comprising: an x-ray beam shaping means comprising a radio-opaque tube having an axially extending orifice of substantially rectangular cross-section; an adapter including first means for mounting said adapter on an x-ray generating unit and second means for rotatably engaging said x-ray beam shaping means so as to permit rotation of said tube about the axis thereof during x-ray beam orientation; and a film-holding instrument freely movable with respect to said tube, said instrument including a bite block adapted to be held between the teeth of the patient and carry a rectangular piece of x-ray film of substantially the same rectangular proportions as said rectangular cross-section of said orifice, an alignment means connected to said bite block and adapted to extend away from the patient and a positioning means connected to said alignment means a spaced distance from said bite block and having a rectangular frame with a rectangular opening of substantially the same rectangular proportions and orientation as said film such that the long sides of said rectangular film are substantially parallel to the long sides of said rectangular opening and the short sides of said rectangular film are substantially parallel to the short sides of said rectangular opening, said positioning means adapted to be coupled to the end of said tube opposite said adapter to assist in establishing alignment between and the same rectangular orientation for said orifice cross-section, said rectangular opening in said positioning means and said film as said tube is rotated thereby substantially confining the beam to strike said film to avoid unnecessary x-ray exposure of the patient.
 6. The dental radiographic system of claim 5 wherein said positioning means comprises keying means for maintaining alignment between the sides and corners of said rectangular frame and said tube orifice rectangular cross-section through positive engagement between said positioning means and said beam shaping means.
 7. The dental radiographic system of claim 5 wherein said x-ray beam shaping means comprises keying means for maintaining alignment between the sides and corners of said rectangular frame and said tube orifice rectangular cross-section through positive engagement between said positioning means and said beam shaping means.
 8. The dental radiographic system of claim 5 wherein both said x-ray beam shaping means and said positioning means comprises keying means for maintaining alignment between the sides and corners of said rectangular frame and said tube orifice rectangular cross-section through positive engagement between said positioning means and said beam shaping means.
 9. The dental radiographic system of claim 8 wherein said keying means comprises at least one notch in said tube and at least one projection extending from said frame member and adapted to be received by said notch to provide alignment between said rectangular opening in said frame member and said tube orifice rectangular cross-section.
 10. The dental radiographic system of claim 9 wherein said frame member includes a pair of said projections extending inwardly from opposite sides of said rectangular opening in said frame member and said tube includes a pair of notches on opposite sides of said tube orifice.
 11. The dental radiographic system of claim 5 wherein said x-ray beam shaping means further comprises a lead diaphragm having a rectangular opening at the end of said tube remote from the patient to form a beam of rectangular cross-section and said tube comprises a lead lining. 